The present invention relates to a method and apparatus for milling a workpiece blank, the workpiece blank rotating in a non-circular manner, and, in particular, for milling a trochoidal rotor.
Trochoidal rotary expansible chamber devices, such as the Wankel or epitrochoidal rotary machines, generally comprise a housing defining a cavity in which is mounted a rotor rotatable in a planetating fashion. Trochoidal rotary devices may be divided into two groups referred to as inner envelope and outer envelope types. In an inner envelope configuration, the profile of the housing cavity is the trochoidal curve and the peripheral profile of the rotor is the inner envelope of the trochoidal curve. In an outer envelope device, the rotor profile is the trochoidal curve and the housing cavity profile is the outer envelope of that curve. Variable spaces formed between facing peripheral surfaces of the rotor an housing cavity serve as working chambers for expansion engines, compressors, expanders, meters, etc. The working chambers are sealed with radially extending apex seals positioned along intersection lines between adjoining peripheral faces on the envelope curve surface.
As those skilled in the art will recognize, the terms "inner envelope" and "outer envelope" refer to the manner in which working member profiles are generated for trochoidal rotary expansible chamber devices. Typical forms of trochoidal devices have fixed housing members for containing rotors travelling in a planetating rotary fashion therein. The known forms have either inner rotors in the form of epitrochoid or hypotrochoid curves or they have inner rotors in the form of envelopes derived from those curves. The designations "epitrochoid" and "hypotrochoid" refer to the manner in which a trochoidal machine's profile curves are generated as described in the Bonavera U.S. Pat. No. 3,117,561.
The manner in which trochoidal curves are formed is well-known in the art. The instant invention applies to all forms of rotary trochoidal machines; however, for purposes of illustration, the invention will be described with reference to epitrochoidal-type machines.
One way to form an epitrochoidal curve is by first selecting a base circle and a generating circle having a diameter greater than that of the base circle. The base circle is placed within the generating circle so that the generating circle is able to roll along on the circumference of the base circle. A curve which is a parallel expansion or a parallel contraction of the epitrochoid is a locus of points, which are equal distance from and normal to the locus of points forming the epitrochoid. The distance between said curves is commonly called seal radius "SR". Curves which are a parallel contraction or expansion of a trochoid or an epitrochoid are commonly referred to as a trochoid or epitrochoid respectively. The distance between the centers of the base and generating circles is conventionally referred to as the eccentricity "e" of the epitrochoidal machine. The epitrochoidal curve is defined by the locus of points traced by the tip of the radially extending generating or drawing arm, fixed to the generating circle and having its inner end pinned to the generating circle center, as the generating circle is rolled about the circumference of the base circle which is fixed. The envelopes are generated by holding the generating circuit stationary and rolling the base circle, carrying the epitrochoid curve with it, about the interior circumference of the generating circle. The inner envelope is the inner outline of the path made by the moving epitrochoid; and the outer envelope is the outer outline of this path. In a typical "inner envelope" epitrochoidal device, the rotor is defined by the envelope profile and rotates in the relationship of the generating circle rolling around the base circle. In an "outer envelope" epitrochoidal device, the rotor is defined by the epitrochoidal curve profile such as that the rotor rotates in the relationship of the base circle rolling around the generating circle.
As is well known in the art of trochoidal devices having a trochoidal rotor of an epitrochoid type, the epitrochoidal configuration is defined by certain mathematical relationships. As described above in the epitrochoid is produced by rolling a generating circle having a radius r around a base circle having a radius r. The term, R, is then defined as the distance from the center of the rolling circle t the tip of the radially extending generating or drawing arm.
Given these fundamental proportions of an epitrochoid a factor K, is defined as K =R/e and an angle of obliquity or leaning angle, .phi., is derived from the relationship ##EQU1## where Z is the number of lobes on the envelope or the number of apexes.
One skilled in the art can now derive the mathematical equations which define each point on an epitrochoid with reference to a selected coordinate system. These equations can then be utilized in cutting or forming a trochoidal rotor from a rotor blank workpiece.
Prior art methods and devices for milling trochoidal rotors are expensive and complex. The epitrochoidal configuration of the rotor requires a milling machine having a high degree of precision. One procedure known in the prior art is to utilize a numerically controlled vertical milling machine for milling the rotor, such as a Gorton Tapemaster Type 2-30 vertical milling machine fitted with a Bunker Ramo Model 3100 three axis numerical control system. Control is provided for the milling table, cross slide and knee elements of the machine tool by means of hydraulic position servo mechanisms driving ball screws. Position feed-back signals are derived from pulse generators mounted on each of the ball screws which feed incremental counters in the control console. A single positioned read-out panel is provided on the console which may be switched to read in any axis.
Although classed as a continuous path machine, it is in fact a digital incremental point-to-point unit with on-line computing facilities which provide circular arc and mirror image machining capability. The incremental position feed-back system enable the machine to be zeroed at any point within its operating range, thereby facilitating setting up procedures.
Programs are prepared on punched paper tape which are derived from computer software, such as APT (Automatically Programmed Tool). The APT program allows the mathematical equations for the article to be milled to be inputted into a computer containing the APT program. The program then produces the punched paper tape for use by the numerical control system on the milling machine. Producing rotors utilizing this prior art method and apparatus is both time consuming and expensive.
The present invention overcomes these drawbacks of CNC machining methods, including those typically illustrated by the above, although these are not necessarily the most current. The present invention provides an effective, economical and simple method and device for milling trochoidal rotors.